1. Field of the Invention
The present invention relates to a temperature sensor for a heating apparatus such as a portable cooking stove or gas range, and more particularly relates to a temperature sensor for measuring the temperature of a container such as a pot or pan placed on and heated by the heating apparatus, so as to control the combustion in the heating apparatus.
2. Description of the Prior Art
In recent years, combustion control systems for a heating apparatus such as a portable cooking stove or gas range have been greatly developed. In the combustion control for a heating apparatus, the temperature of a container such as a pot or pan placed on the heating apparatus is measured, so that the combustion in the heating apparatus is controlled in accordance with the measured temperature. With a combination of thermometry and electronic control technique in the combustion control technique, a heating apparatus including a combustion control system has become easier to operate.
Temperature sensors currently used for such a combustion control system include a sensor using a thermal element made of a semiconductor such as a ceramic, a sensor utilizing the expansion coefficient of a liquid, a bimetal made of different metals which allows electric contacts to open or to close, etc.
There are a variety of thermometric systems using temperature sensors. For example, in some thermometric systems, a temperature sensor is brought into contact with a pot or pan placed on a heating apparatus to measure the temperature thereof. In other systems, a temperature sensor is brought into contact with the food contained in a pot or pan placed on a heating apparatus so as to measure the temperature thereof.
With the development of such a temperature sensor, there has been an increasing demand for a heating apparatus having excellent operability and durability. However, a conventional temperature sensor having a thermal element of a ceramic such as SiC has a complicated structure and accordingly requires high production costs. Furthermore, the durability of this temperature sensor is poor because the ceramic used for the thermal element has a tendency to break on receiving thermal or mechanical shock. When this temperature sensor is incorporated into a heating apparatus, the lifetime of the heating apparatus becomes short because of the poor durability of the temperature sensor. A conventional temperature sensor utilizing the expansion coefficient of a liquid also has the disadvantage of having poor response to changes in temperature. When this temperature sensor is incorporated into a heating apparatus, the heating apparatus becomes difficult to operate because the temperature sensor has poor response to changes in the temperature of the container to be heated.
Thus, there is a great demand for an inexpensive heating apparatus which includes a temperature sensor and has a long lifetime and excellent operativity.
Fig. 1 shows a conventional temperature sensor having a thermal element made of a ceramic such as SiC, which is incorporated in a heating apparatus 1. The temperature sensor 52 has a rigid cylindrical body 53 and a plate 54 mounted on one end of the rigid cylindrical body 53. An electrical insulator 55 is brazed to the lower face of the plate 54. A thermal element 56 is fixed to the electrical insulator 55 by means of a glass sealing 57. In this arrangement, the thermal element 56 is electrically insulated from the rigid cylindrical body 53 and the plate 54. A pair of leads 59 each coated with an electrical insulator 58 are connected to the thermal element 56 and extend downward from the glass sealing 57.
Since the thermal element 56 is electrically insulated from the rigid cylindrical body 53, it is not influenced by any noise generated from the discharge of an ignition device (not shown) for turning on a burner (not shown) of the heating apparatus 1, or from the electromotive force of a burner safety device (not shown). If the thermal element 56 is influenced by such a noise, the data output from the temperature sensor 52 becomes inaccurate, which causes malfunction of the heating apparatus 1.
The reference numeral 10 denotes a container to be heated by the heating apparatus 1. The plate 54 is in contact with the container 10 to conduct heat therefrom to the thermal element 56. The thermal element 56 then measures the temperature of the container 10 and sends data containing the measured result to a control system (not shown) of the heating apparatus 1 through the leads 59. On receiving the data, the control system controls the combustion in the heating apparatus 1.
When the container 10 is used for cooking tempura (Japanese deep-fried food) or the like, the temperature thereof reaches about 220.degree. C. When the container 10 is a grill pan or frying pan on which food is grilled or broiled, the temperature thereof increases to about 280.degree. C. In cases where the insulator 55 is made of a ceramic or the like, it is resistant to such high temperatures. However, when the container 10 is heated to such high temperatures, there arises difference in temperature among the plate 54, the insulator 55, the glass sealing 57 and the thermal element 56. This causes the respective components to have different expansion coefficients, which in turn brings about stress in the temperature sensor 52. The stress causes the glass sealing 57 which has poor mechanical strength to break.
Furthermore, the insulator 58 on each lead 59 is usually made of fluororesin or the like which is resistant to temperature of less than about 250.degree. C. When the container 10 is heated to 250.degree. C. or more, the insulator 58 is melted, which often causes the leads 59 to come into contact with each other and to be short-circuited. As a result, the data sent out from the temperature sensor 52 through the leads 59 become inaccurate. The inaccurate data makes it impossible to accurately control the combustion in the heating apparatus 1. Therefore, it is impossible to cook food at a temperature of 250.degree. C. or more with use of the heating apparatus 1 including the temperature sensor 52.